In semiconductor fabrication, various layers of insulating material, semiconducting material and conducting material are formed to produce a multilayer semiconductor device. The layers are patterned to create features that taken together, form elements such as transistors, capacitors, and resistors. These elements are then interconnected to achieve a desired electrical function, thereby producing an integrated circuit (IC) device. The formation and patterning of the various device layers are achieved using conventional fabrication techniques, such as oxidation, implantation, deposition, epitaxial growth of silicon, lithography, etching, and planarization.
In the semiconductor manufacturing process several different devices employ doping of a particular region to alter its electrical characteristics. For example, the well known complimentary metal oxide semiconductor (CMOS) technology in manufacturing, for example, relies on doping to manufacture n-channel (NMOS) and p-channel (PMOS) structures that are widely used in manufacturing for example, transistor and memory devices such as dynamic random access memory (DRAM) and static random access memory (SRAM). As device features become increasingly smaller, the required doping specifications are increasingly stringent.
Although there have been several electrical measurement methods proposed to measure doping profiles of semiconductor structures such as, for example, Current-Voltage (CV) methods, these methods are unable to determine doping concentration as a function of depth with sufficient resolution. Other methods have also had limited success in obtaining a doping concentration profile of sufficient resolution as a function of depth, leaving the best potential method in terms of element detection limit and depth resolution as secondary ion mass spectroscopy (SIMS). SIMS is the method of choice for surface and near surface investigation of solid samples because of its ability to provide parts-per-million to parts-per-billion sensitivity and excellent depth resolution.
SIMS is a well known a technique for surface and near surface analysis which involves ion bombardment of the sample surface for depth profiling. Primary ions are accelerated by a voltage bias to bombard the target surface where they are implanted, resputtered, and produce sputtered secondary ions from the target material. The secondary ions are separated depending on mass and detected and counted by a detector. As the process proceeds, the target surface is gradually removed according to a sputter rate depending on, for example, primary beam intensity, sample material, and crystal orientation. Typical sputter rates of 2-5 Angstroms per second, at data acquisition time intervals of 0.5-10 seconds, produce typical depth increments in the 20-50 Angstrom range. The primary ion beam species that are typically useful in SIMS analysis include Cs+, O2+, O−, Ar+, and Ga+ at energies between 1 and 30 keV.
The sputtered ions are collected by a mass spectrometer for mass to charge separation and detection. The number of ions collected can also be digitally counted and integrated to produce quantitative data on the sample composition. By monitoring the secondary ion signals with time (sputter depth), a dopant concentration depth profile can be produced.
One problem with applying SIMS in the analysis of semiconductor structures is the large analytical area of about 100 microns by about 100 microns. As a result, SIMS has not been able to measure doping profiles of individual semiconductor structures as a function of depth (i.e., a two-dimensional profile). As such, the use of SIMS has been largely restricted to one dimensional analysis in the semiconductor art, where the surface dopant concentration over the analytical area is determined.
One example where a two dimensional doping concentration analysis is required is in a lightly doped drain (LDD) structure. In scaling down devices, thinner gate oxide and more highly doped channels are needed to increase the punch-through voltage. As a result, the electric field near the drain regions can cause hot charge carriers to penetrate the oxide barrier into the gate thereby degrading device performance.
An LDD structure alleviates this problem by including a lightly doped region near the gate while allowing a heavier doping concentration in the region further from the gate. Generally described, an LDD structure in an NMOS or PMOS device includes a gate with oxide sidewall spacers. The LDD region is below the sidewall spacers adjacent to, for example in an NMOS device, a P doped substrate known as a P-well. The doping region between oxide gates is first subjected to a lighter doping dose of for example, phosphorous, prior to forming the oxide sidewall spacers adjacent to the gate structure. The sidewall spacers act to shield a portion of the previously doped doping region while the doping region is again subjected to a second heavier doping dose of, for example, arsenic. The two doping regions formed include a more lightly doped region near the gate structure under the sidewall spacers and a more highly doped region displaced from the gate edge. Thus, the electric field is reduced near the gate thereby alleviating the problem of hot charge carrier penetration into the gate.
While there are several methods for carrying out doping by, for example, implantation methods, as feature sizes shrink, channeling of implanted ions is more likely to adversely affect device performance making it critical to carry out implanting at the proper energies and intensities. As such, device performance depends heavily on gauging the precision in the level of doping and its spatial profile. As yet, no satisfactory methods have been developed to allow the use of SIMS to analyze the doping profiles of individual submicron semiconductor features in two dimensions.
There is therefore a need in the semiconductor processing art to develop a method and apparatus whereby two dimensional doping profiles using SIMS can be obtained for individual semiconductor features thereby allowing doping concentrations to be spatially determined (doping profile) with improved precision in a semiconductor device structure doping region.
It is therefore an object of the invention to provide a method and apparatus whereby two dimensional doping profiles for semiconductor device structures using SIMS can be determined thereby allowing doping concentrations to be spatially characterized with improved precision in a semiconductor device doping region while overcoming other shortcomings and deficiencies in the prior art.